BMR Explained: How Basal Metabolic Rate Is Calculated and Used

What is BMR?

Basal metabolic rate, or BMR, is the energy your body burns when it is doing nothing — lying flat, awake but completely still, in a warm room, with nothing in the stomach and no recent exercise. It is the bill the body pays just to stay alive: keeping the heart beating, the kidneys filtering, the brain firing, the liver running its thousand reactions a second, and the core temperature held within tenths of a degree of 37 °C. For a healthy adult, that base bill is roughly 1,200 to 2,000 kilocalories a day, depending almost entirely on body size and lean mass. The BMR calculator on this page estimates yours from your sex, age, height, and weight.

The number matters because it is the floor from which every other calorie target is built. Daily maintenance, the appropriate cut for fat loss, the right surplus for lean muscle gain — all of them start with BMR and then layer activity on top. Use the wrong BMR and every downstream target is wrong by the same proportion. That is also why the equation that the calculator uses matters more than the decimal places in the answer; a 5% error in the formula propagates into a 5% error in maintenance calories, which across a year is roughly four kilograms of fat in either direction.

BMR is sometimes used interchangeably with RMR (resting metabolic rate). They are close but not identical. BMR is measured under strict laboratory conditions — fasted, supine, awake, thermoneutral, no recent activity. RMR is measured under more relaxed conditions and typically runs about 10% higher. The Mifflin-St Jeor and Harris-Benedict equations used by the BMR calculator predict BMR specifically; if a fitness tracker or app says “RMR,” expect a slightly larger number for the same person.

How BMR is calculated

The modern reference equation is Mifflin-St Jeor, published in 1990 by Mark Mifflin and Sachiko St Jeor in the American Journal of Clinical Nutrition. It is the formula the Academy of Nutrition and Dietetics endorsed in its 2014 evidence analysis as the most accurate of the major predictive equations for healthy adults. In SI units it is:

BMR (men)   = 10 × kg + 6.25 × cm − 5 × age + 5 BMR (women) = 10 × kg + 6.25 × cm − 5 × age − 161

Read those constants aloud and they tell a story. Every kilogram of body mass adds about 10 kcal/day to the rest bill. Every centimetre of height adds about 6.25 kcal/day, because taller people carry more organ mass and more skin to keep warm. Every year of age subtracts 5 kcal/day, mostly because lean tissue is gradually replaced by less expensive fat. And the flat 166 kcal/day gap between men and women is what is left after weight and height are accounted for — an offset that captures the average difference in body composition between the sexes at the same height and weight.

The older Harris-Benedict equation, originally published in 1919 and revised by Roza and Shizgal in 1984, is the one many fitness apps and printed nutrition tables still use:

BMR (men)   = 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age BMR (women) = 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age

Harris-Benedict tends to overestimate BMR by about 50 to 100 kcal/day relative to Mifflin-St Jeor in modern populations, partly because the original 1919 sample was atypically lean and active by today’s standards. The BMR calculator shows the Harris-Benedict value alongside Mifflin-St Jeor so older plans and recommendations can be compared on the same page, but uses the Mifflin-St Jeor figure as the primary output.

For imperial inputs the calculator converts using the NIST-exact factors of 1 pound = 0.45359237 kg and 1 inch = 2.54 cm, then applies the metric formula. The answer is identical whichever unit system you choose; small disagreements between online calculators almost always come down to rounded conversion factors rather than a different underlying equation.

Worked example

Take a 30-year-old man, 80 kg, 180 cm tall. The Mifflin-St Jeor calculation is:

BMR = 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 800 + 1125 − 150 + 5 = 1,780 kcal/day

That is 1,780 kilocalories burned to do absolutely nothing for 24 hours. Per hour, 1,780 ÷ 24 ≈ 74 kcal, roughly the energy in a slice of buttered toast or half a banana, every hour, just to keep him alive. The BMR calculator reports the per-hour figure as a sanity check, because it makes the size of the resting bill more intuitive than the daily total.

For comparison, the Harris-Benedict revised value for the same person is 88.362 + 13.397 × 80 + 4.799 × 180 − 5.677 × 30 = 1,853 kcal/day, about 73 kcal higher. That gap is typical and is one of the practical reasons newer dieticians and sports nutritionists prefer Mifflin-St Jeor: the lower predicted BMR leads to slightly more aggressive deficits and surpluses, which more often match what people actually need in practice.

Now take a 45-year-old woman, 65 kg, 165 cm tall. Mifflin-St Jeor gives 10 × 65 + 6.25 × 165 − 5 × 45 − 161 = 650 + 1031 − 225 − 161 = 1,295 kcal/day. Per hour, that is about 54 kcal. To estimate the calories she actually needs each day, multiply by an activity factor — that calculation is BMR × activity, which the TDEE calculator does automatically.

Factors that affect BMR

Lean body mass

Skeletal muscle, organ tissue, and bone are metabolically far more expensive than fat. At rest, a kilogram of muscle burns roughly 13 kcal/day, a kilogram of fat about 4 kcal/day, and the major organs (liver, kidneys, brain, heart) burn 200 to 400 kcal/day per kilogram of tissue, despite making up a small fraction of total body mass. Two people the same height and weight but with very different muscle mass can have BMRs that differ by 200 kcal/day or more. The predictive equations cannot see body composition, so they will overestimate BMR for very lean adults and underestimate it for very muscular ones. If you carry an unusual amount of lean mass, the Katch-McArdle equation (which uses lean mass directly) is usually a better fit, and the lean body mass calculator can give you the input it needs.

Age

BMR drops with age in healthy adults, partly through sarcopenia — the gradual loss of lean muscle mass starting in the fourth decade of life — and partly through declining organ activity. The 5 kcal/day per year of age in Mifflin-St Jeor captures the average rate of decline for most of adulthood. A 2021 paper in Science (Pontzer and colleagues) measured energy expenditure with doubly labelled water across 6,400 people and found that per-kilogram BMR is actually flat from roughly age 20 to 60, then declines about 0.7% per year after age 60. The predictive equations smooth that into a single linear slope, which slightly overestimates BMR for people in their 20s and 30s and slightly underestimates it for people beyond 60.

Sex

At the same height and weight, men typically carry more skeletal muscle and less essential body fat than women, and as a result burn around 166 kcal/day more at rest. That is the gap baked into the Mifflin-St Jeor constants. It is an average, not a per-person rule. A lean, well-trained woman can have a true BMR that is higher than a sedentary man of the same weight, simply because she carries proportionally more lean mass. The flat sex offset works well for the middle of the population and increasingly poorly out at the extremes of body composition.

Thyroid and hormonal status

Thyroid hormone is the body’s metabolic rheostat. Mild hyperthyroidism can lift measured BMR by 10 to 30% above prediction; hypothyroidism can pull it 10 to 40% below. Cortisol, growth hormone, leptin, and ghrelin all modulate resting expenditure on shorter timescales. The predictive equations assume an average endocrine state. People with diagnosed thyroid disease, recent severe dieting, or long-term very low-calorie intakes can sit well outside the prediction interval — usually low. Indirect calorimetry is the right tool when a clinical reason makes accurate BMR genuinely important.

Body temperature and environment

BMR rises about 7% for every 1 °C the core body temperature climbs, which is why a fever materially increases calorie needs. Cold ambient temperatures push BMR up modestly through non-shivering thermogenesis in brown adipose tissue, particularly in younger adults. Sustained living in hot climates appears to drop BMR by a few percent. These effects are small in day-to-day life and not captured by the predictive equations, which assume a thermoneutral environment.

How to use the BMR number

• Treat it as a starting point, not a verdict. Predictive BMR is within ±10% of measured BMR for roughly 70% of healthy adults. That is good enough to plan from, not good enough to argue with the scale about after two weeks. Set a calorie target, eat to it for three or four weeks, then adjust based on the actual weight trend.
• Build TDEE on top, do not eat at BMR. BMR is the bill for breathing. You also walk, type, climb stairs, fidget, and digest food — activities that add another 20 to 100% on top, depending on lifestyle. Multiplying BMR by the appropriate activity factor with the TDEE calculator gives the figure you should actually eat to maintain weight.
• Set deficits and surpluses against TDEE, not BMR. A typical fat-loss deficit is 300 to 500 kcal/day below TDEE, producing 0.25 to 0.5 kg of weekly loss. A typical lean-bulk surplus is 150 to 300 kcal/day above TDEE. Both targets sit comfortably above BMR for any healthy adult; eating below BMR for more than a few days is rarely necessary and risks the body adapting downward to defend its weight.
• Recalculate after meaningful weight change. BMR scales with weight, so a person who has lost or gained 10 kg has a different BMR than the one they started with. Re-run the BMR calculator every few kilograms, not every day. Daily changes are noise.
• Resistance-train to preserve the floor. The single most effective thing an adult can do to keep BMR from drifting downward through middle age is to maintain skeletal muscle. Two to three resistance sessions a week through the 40s, 50s, and 60s prevents most of the sarcopenia-driven decline, and the preserved muscle directly raises BMR by 10 to 15 kcal/day per additional kilogram.
• Use Katch-McArdle if you know your body fat. The predictive equations on this page use total weight. If you have a recent DEXA or accurate body-fat estimate, Katch-McArdle (BMR = 370 + 21.6 × lean kg) is usually more accurate, particularly for very lean or very muscular adults outside the population that Mifflin-St Jeor was fit to.

Common mistakes

Confusing BMR with TDEE

BMR is the rest bill. TDEE is the rest bill plus everything else — walking, training, digesting, thinking, fidgeting. Eating at BMR means under-eating by 20 to 100% for any normally active adult. Many crash-diet plans accidentally target BMR rather than TDEE because the two numbers are confused, which is the fastest way to provoke the metabolic adaptation that ruins a fat-loss plan.

Trusting one decimal place

A predictive BMR of 1,784 kcal/day is not meaningfully different from 1,800 or 1,770. The 95% prediction interval around any Mifflin-St Jeor estimate is roughly ±200 kcal/day. Round to the nearest 50 kcal, set a target, and adjust based on observed scale movement. Chasing decimals wastes time that should be spent looking at the trend.

Using adult equations on children or adolescents

Mifflin-St Jeor and Harris-Benedict were fit on adult populations and produce nonsense for under-18s. The Schofield equations and the FAO/WHO/UNU 2004 child equations are the standard references for children and adolescents; the WHO publishes age- and sex-specific tables for clinical use. The BMR calculator restricts age to 14 and above for this reason and is best used from age 18.

Comparing values across different equations

Mifflin-St Jeor, Harris-Benedict, Katch-McArdle, the Schofield set, the WHO/FAO/UNU equations, and the Cunningham equation all give different numbers for the same person. That is not a sign one of them is broken; they were fit to different populations under different assumptions. Pick one equation, use it consistently, and adjust based on real-world results. Mixing equations across a single plan produces phantom progress or apparent setbacks that are really just formula differences.

When to seek professional advice

Predictive BMR is good enough for everyday calorie planning. There are a handful of situations where it is worth asking a clinician or dietitian for a more accurate measurement rather than relying on the equation:

• Unexplained weight gain or weight loss in spite of stable eating and activity, which can be a sign of thyroid disease or another endocrine condition that materially shifts BMR.
• Long-term very low-calorie dieting or a history of eating disorders, both of which can suppress measured BMR well below predicted — sometimes by 20 to 30% — and require careful refeeding rather than aggressive deficits.
• Pregnancy, lactation, or post-partum recovery, where calorie needs are different from the predictive equations and are best set against guidance from a midwife or obstetric dietitian.
• Competitive athletes preparing for events where calorie precision matters more than typical, particularly endurance sports and physique sports. Indirect calorimetry through a sports nutritionist or university lab is the gold standard.
• Children, adolescents, and adults over 75, where the standard predictive equations are least accurate.

Frequently asked questions

What is the difference between BMR and TDEE? BMR is the calories you would burn lying completely still in a thermoneutral room for 24 hours, fasted — pure organ and tissue maintenance. TDEE adds in everything else: walking, training, digesting, fidgeting, thinking. TDEE is BMR multiplied by an activity factor, and is the figure you should actually eat to maintain weight. For most adults BMR is roughly 60 to 70% of TDEE; a sedentary office worker sits around BMR × 1.2, a moderately active person around BMR × 1.55. The TDEE calculator handles the multiplication.

Which BMR equation is most accurate? For healthy adults, Mifflin-St Jeor (1990) is the modern reference. The Academy of Nutrition and Dietetics 2014 evidence analysis compared the major predictive equations against indirect calorimetry and found Mifflin-St Jeor was correct within ±10% for about 70% of people, beating Harris-Benedict and the WHO/FAO/UNU equations. For very lean or very muscular adults the Katch-McArdle equation, which uses lean mass directly, is usually more accurate. For older adults beyond about 75, none of the predictive equations are reliable and indirect calorimetry is recommended where it matters.

Can I lose weight by eating below my BMR? You can, but it is rarely a good idea. Sustained eating below BMR triggers metabolic adaptation: the body slows non-essential systems (thyroid output, reproductive hormones, immune function) to defend its weight, which makes further loss harder and risks muscle loss, fatigue, and nutrient deficiencies. Standard guidance is to set a moderate deficit of 300 to 500 kcal/day below TDEE, which produces 0.25 to 0.5 kg of weekly loss without forcing the body into a stress response. Plans that drop a person below BMR for weeks at a time are why so much rapid-loss dieting plateaus, then rebounds.

Does BMR change with weight loss? Yes — BMR drops as weight drops, because a smaller body has less tissue to maintain. Some of the drop is what the Mifflin-St Jeor formula predicts from the lower weight alone; some is adaptive thermogenesis, an additional 50 to 200 kcal/day suppression that persists for months or years after a significant loss. This is one reason why the calorie target that produced weight loss in the first place stops working after a few months, and why re-running the BMR calculator with the new weight, then recalculating TDEE, is necessary every 4 to 5 kilograms of loss.

Why are imperial and metric BMR results sometimes slightly different? They should not be. The calculator converts pounds to kilograms with the NIST-exact factor 0.45359237 and inches to centimetres with 2.54, then applies the metric Mifflin-St Jeor formula. Any small disagreement between an online imperial calculator and an online metric one comes from one of them using a rounded conversion factor (often 0.4536 or 0.454), not from a different underlying equation.

Should I use a different BMR equation as I age? The standard Mifflin-St Jeor formula models age decline as 5 kcal/day per year, a single linear slope. Recent doubly-labelled-water work (Pontzer et al., Science, 2021) suggests that per-kilogram BMR is roughly flat from 20 to 60 and only really declines after 60, more steeply than the linear slope predicts. For adults aged 60 to 75 the equation works adequately. Beyond 75 it tends to overestimate, and indirect calorimetry is the right tool where accuracy genuinely matters.

Does muscle really burn that many more calories than fat? At rest, a kilogram of skeletal muscle burns about 13 kcal/day and a kilogram of fat about 4 kcal/day — roughly three times the rate, but in absolute terms still small. Adding 5 kg of muscle through serious training adds only about 45 kcal/day to BMR, not the hundreds sometimes quoted by fitness marketing. The real metabolic value of training is not the resting calorie boost; it is the higher TDEE on training days, the better insulin sensitivity, and the preservation of lean mass through middle age.

How accurate are predictive BMR equations versus measured BMR? Within ±10% for roughly 70% of healthy adults. The remaining 30% sit further off, usually because of unusual body composition, thyroid disease, recent severe dieting, or being outside the age range the equation was fit to. The gold standard is indirect calorimetry, where gas exchange is measured under a hood or in a chamber. Calorimetry is typically only available in research labs and specialist clinics; for everyday calorie planning, the predictive value is a perfectly serviceable starting point.

Related calculators

Use these alongside the BMR calculator to turn the resting figure into a working calorie plan.

• TDEE calculator — multiplies BMR by an activity factor to give the total daily calories you actually burn, which is the figure you should eat to maintain weight.
• Calorie calculator — maintenance, cut, and bulk calorie targets, with the deficit or surplus already applied so you can set a target directly.
• BMI calculator — body mass index with the WHO weight categories, a population-level screening tool that pairs naturally with BMR for setting weight goals.
• Lean body mass calculator — estimate muscle, bone, and organ mass from height, weight, and sex, the input the Katch-McArdle equation needs for a body-composition-aware BMR.
• Weight converter — convert between kilograms, pounds, stones, and ounces if your scale and your training app disagree on units.